What is a suitable incipit for Victoria's Laudate Pueri? As usually found in the scores of this piece, the setting begins with "Laudate pueri Dominum..." but it omits the opening of the Psalm 112/113 text, which begins with the word "Alleluia." What Alleluia incipit would you recommend, or have you heard before?Thanks.

The Halleel psalms (112 through 117, or other numberings) are psalms of praise, so there is something to be said for "Alleluia" to be more of a title than an actual part of the text. Omitting it would make sense in this reading.

Should you want to include it, any antiphonary is bound to have loads of Alleluia antiphons, so you'd just have to pick your choice.

Victoria's piece is not incomplete, but an intonation is an elegant way to give the starting pitch. At vespers Laudate pueri would be sung with the antiphon Sit nomen Domini (p. 259 in my edition of the Liber Usualis), which alternatively could be said softly while the organ played.